The present invention relates to a semiconductor photodetector, and more particularly to a semiconductor photodetector with an increased photo receiving area and exhibiting high speed performances.
Semiconductor waveguide photodetectors have been known as one kind of the photodetectors in the art to which the present invention pertains. One of the conventional semiconductor waveguide photodetectors is disclosed in Japanese laid-open patent publication No 4-268770. FIG. 1 is a schematic perspective view illustrative of the conventional semiconductor waveguide photodetector. The structure of the conventional semiconductor waveguide photodetector. A semiconductor substrate, on which the conventional semiconductor waveguide photodetector is formed, comprises an n.sup.+ -InP substrate 11 having a carrier concentration of 1.times.10.sup.18 cm.sup.-3. A bottom cladding layer 12 is provided on a top surface of the n.sup.+ -InP substrate 11, wherein the bottom cladding layer 12 comprises an n.sup.+ -In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y (x=0.33, y=0.7) bottom cladding layer 12 having a carrier concentration of 1.times.10.sup.18 cm.sup.-3 and a thickness of 2 micrometers as well as having a band gap wavelength of 1.37 micrometers. A core layer 13 is provided on a top surface of the n.sup.+ -In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y (x=0.33, y=0.7) bottom cladding layer 12, wherein the core layer 13 comprises an n.sup.+ -In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y (x=0.35, y=0.76) core layer 13 having a carrier concentration of 1.times.10.sup.18 cm.sup.-3 and a thickness of 0.1 micrometer as well as having a band gap wavelength of 1.42 micrometers. A top cladding layer is provided on the n.sup.+ -In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y (x=0.35, y=0.76) core layer 13. The top cladding layer comprises a first top cladding layer 14 extending on an entire surface of the n.sup.+ -In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y (x=0.35, y=0.76) core layer 13, a second top cladding layer 15 selectively extending on a predetermined region of a top surface of the first top cladding layer 14, and a third top cladding layer 16 extending on an entire surface of the second top cladding layer 15, so that the second and third top cladding layers 15 and 16 form a ridged portion. The laminations of the first and second top cladding layers 14 and 15 comprise 20 periods of alternating laminations of undoped n.sup.+ -In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y (x=0.42, y=0.9) layers having a thickness of 0.005 micrometers and having a band gap wavelength of 1.57 micrometers and undoped In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y (x=0.33, y=0.7) layers having a thickness of 0.01 micrometers and having a band gap wavelength of 1.29 micrometers. The third top cladding layer 16 comprises a p.sup.+ -In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y (x=0.33, y=0.7) layer having a thickness of 2 micrometers and a carrer concentration of 1.times.10.sup.18 cm.sup.-3. An n-electrode 17 is provided on an entire bottom surface of the n+-InP substrate 11. A p-electrode 18 is provided on an entire bottom surface of the third top cladding layer 16. The laminations of the n+-InP substrate 11, the n.sup.+ -In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y (x=0.33, y=0.7) bottom cladding layer 12, and the n.sup.+ -In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y (x=0.35, y=0.76) core layer 13 are in the form of an n-electrode side region. The laminations of the first and second top cladding layers 14 and 15 are in the form of a low carrier concentration intermediate region. The third top cladding layer 16 is in the form of a p-electrode side region. The above undoped In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y (x=0.42, y=0.9) layers of 0.005 micrometers in thickness and of 1.57 micrometers in band gap wavelength but only in the second top cladding layer 15 in the ridged portion are capable of absorbing a light having a wavelength of 1.55 micrometers to cause a photoelectric transfer. The light having the wavelength of 1.55 micrometers can be absorbed by only the undoped In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y (x=0.42, y=0.9) layers. This means that the photo receiving area of the above conventional device is small and thus a coupling efficiency of the photodetector is low.
In the above circumstances, it had been required to develop a novel semiconductor photodetector free from the above problems.